1. Field of the Invention
The present invention relates to a scanning apparatus for reading an original image of a copying machine.
2. Description of the Prior Art
The scanning apparatus for reading the original image of an original-fixed type copying machine is designed to optically scan an original placed on an original table by an exposure unit of the apparatus, thereby a latent image is produced on a photoreceptor of a recording part of the copying machine. The exposure unit usually consists of the following two moving frames: a first moving frame including an exposure light source such as an exposure lamp which emits light toward the original table, and a reflecting member such as a mirror which reflects the light reflected at the original in a predetermined direction; and a second moving frame including a reflecting member such as a mirror which reflects the light reflected by the reflecting member of the first moving frame in a direction where a lens is positioned.
Conventionally, in order to scan the original, a mechanism having a wire and pulleys (hereinafter, referred to as "wire and so on") has been mainly adopted as drive transmitting means which moves the first and second moving frames. However, in the case of transmitting drive power of a motor by using the wire and so on, it is easy for slipping to occur, further, if the wire is wound by many turns to prevent this, a problem is produced that assembling work is quite inefficient.
Additionally, if the wire and so on are utilized, it is difficult to hold well-balanced initial tension of the wire on the respective ends of the first and second moving frames. Moreover, the wire tends to be lengthened due to repetition of the scanning operation by the exposure unit, therefore it becomes necessary to make adjustment again because the elongation of the wire is different between the respective end portions due to unevenness in the properties of the matter of the wire. And it requires a lot of time to make such adjustment.
For the purpose of solving the above-described problem, there have been proposed scanning apparatuses in which toothed belts are employed as the drive transmitting members in place of the wires and so on. For example, Japanese Patent Laid-Open Publication No. 110128/1986 discloses such drive transmitting means as shown in FIG. 1. According to this prior art, on each outside of side plates 156 and 157, a toothed belt 168, on which the first moving frame is fixed, is secured at its one end by a fixing member 169, extended to wrap around a toothed pulley 166 installed on the second moving frame to fold back, next, wrapped around a driving pulley 162 to fold back in the original direction, further wrapped around a driven pulley 163, finally wrapped around a toothed pulley 167 installed on the second moving frame together with the toothed pulley 166, and thereafter secured to each side plate by a fixing means 170. Thus, the first and second moving frames are driven by the wrapping connector drive of the toothed belt 168.
In addition, Japanese Patent Laid-Open Publication No. 42142/1987 discloses another construction of the drive transmitting means as shown in FIG. 2. The first moving frame is fixed at each end on a toothed belt 138 which connects a driving pulley 134 and a driven pulley 136, while the second moving frame is fixed at each end on a toothed belt 144 which connects two toothed pulleys 140 and 142 having a different diameter from the driven pulley 136 and the driving pulley 134 respectively. Thus, the first and second moving frames are reciprocated by the drive of the toothed belts 138 and 144 respectively.
However, according to the above-described former prior art, since one portion of the toothed belt 168 has to take a double-toothed shape, it cannot be said to be of practical use due to difficulty in manufacturing such a peculiar shaped toothed a belt. In addition, belt fixing member is required to fix both end portions of the toothed belt 168, and adjustment of the length of the belt is difficult. On the other hand, according to the latter prior art, one belt is needed for one end portion of each moving frame, which not only increases cost because a total of four belts are needed, but also complicates the construction of the apparatus itself, and further, in mass-production, leads to more than a little trouble due to the unevenness in the properties of the matter of the belt.